Reflective projection lens for EUV-photolithography

ABSTRACT

A projection lens for imaging a pattern arranged in an object plane onto an image plane using electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region has several imaging mirrors between its object plane and image plane that define an optical axis of the projection lens and have reflective coatings. At least one of those mirrors has a graded reflective coating that has a film-thickness gradient that is rotationally symmetric with respect to a coating axis, where that coating axis is acentrically arranged with respect to the optical axis of the projection lens. Providing at least one acentric, graded, reflective coating allows designing projection lenses that allow highly uniform field illumination, combined with high total transmittance.

[0001] The following disclosure is based on U.S. Provisional ApplicationNo. 60/308,861 filed on Aug. 1, 2001, which is incorporated into thisapplication by reference and from which priority is claimed.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a projection lens for imaging a patternarranged in an object plane onto an image plane employingelectromagnetic radiation from the extreme-ultraviolet (EUV) spectralregion.

[0004] 2. Description of the Related Art

[0005] Projection lenses of that type are employed on projectionexposure systems used for fabricating semiconductor devices and othertypes of microdevices and serve to project patterns on photomasks orreticles, which shall hereinafter be referred to using the generic terms“masks” or “reticles,” onto an object having a photosensitive coating atultrahigh resolution.

[0006] In order to allow creating even finer structures, variousapproaches to improving the resolving power of projection lenses arebeing pursued. It is well known that resolving power may be improved byincreasing the image-side numerical aperture (NA) of the projectionlens. Another approach is employing shorter-wavelength electromagneticradiation.

[0007] However, improving resolution by increasing numerical aperturehas several disadvantages. The major disadvantage is that the attainabledepth of focus (DOF) decreases with increasing numerical aperture, whichis disadvantageous because, for example, a depth of focus of the orderof at least one micrometer is desirable in view of themaximum-attainable planarity of the substrate to be structured andmechanical tolerances. Systems that operate at moderate numericalapertures and improve resolving power largely by employingshort-wavelength electromagnetic radiation from the extreme-ultraviolet(EUV) spectral region have thus been developed. In the case ofEUV-photolithography employing operating wavelengths of 13.4 nm,resolutions of the order of 0.1 μm at typical depths of focus of theorder of 1 μm may theoretically be obtained for numerical apertures ofNA=0.1.

[0008] It is well known that radiation from the extreme-ultravioletspectral region cannot be focused using refractive optical elements,since radiation at the short wavelengths involved is absorbed by theknown optical materials that are transparent at longer wavelengths.Mirror system that have several imaging, i.e., concave or convex,mirrors that have reflective coatings arranged between their objectplane and image plane and define an optical axis of the projection lensare thus employed in EUV-photolithography. The reflective coatingsemployed are typically multilayer coatings having, for example,alternating layers of molybdenum and silicon.

[0009] A reflective lens for use in EUV-photolithography that has fourmirrors, each of which has reflective coatings with uniformly thicklayers, is disclosed in U.S. Pat. No. 5,973,826.

[0010] Another EUV-photolithographic system is shown in U.S. Pat. No.5,153,898. That system has a maximum of five mirrors, at least one ofwhich has an aspherical reflecting surface. Numerous combinations ofmaterials for multilayer reflective coatings suitable for use in the EUVare stated. Their layers all have uniform thicknesses.

[0011] Although reflective coatings with uniform thicknesses arerelatively simple to deposit, in the case of imaging systems where theangle of entry, or angle of incidence, of the radiation employed onthose areas of the mirrors utilized varies, they usually generate highreflection losses, since the thicknesses of their layers are optimizedfor a specially selected angle of incidence, or a narrow range of anglesof incidence, only. Another of their disadvantages is a nonuniform pupilirradiance that causes a telecentricity error, structurally dependent orfield-dependent resolution limits (so-called “H−V-differences or“CD-variations”), and generally lead to a narrowing down of theprocessing window.

[0012] Reflective EUV-imaging systems that have mirrors that have gradedreflective coatings that are characterized by the fact that they have afilm-thickness gradient that is rotationally symmetric with respect tothe optical axis of the entire system are also known (cf. U.S. Pat. No.5,911,858). Employing graded reflective coatings allows achieving a moreuniform distribution of the reflected intensity over a certain range ofangles of incidence.

[0013] Photolithographic equipment, or steppers, employ two differentmethods for projecting a mask onto a substrate, namely, the“step-and-repeat” method and the “step-and-scan” method. In the case ofthe “step-and-repeat” method, large areas of the substrate are exposedin turn, using the entire pattern present on the reticle. The associatedprojection optics thus have an image field that is large enough to allowimaging the entire mask onto the substrate. The substrate is translatedafter each exposure and the exposure procedure repeated. In the case ofthe step-and-scan method that is preferred here, the pattern on the maskis scanned onto the substrate through a movable slit, where the mask andslit are synchronously translated in opposite directions at rates whoseratio equals the projection lens' magnification.

SUMMARY OF THE INVENTION

[0014] It is one object of the invention to provide an EUV-projectionlens operable at high numerical aperture that will allow largelycorrecting distortion errors along all image directions and providingsufficiently symmetric, high-intensity, illumination of the image field,while maintaining adequate-quality imaging. It is another object toprovide a projection lens that, from the optical standpoint, representsa reasonable compromise among wavefront errors, distortion, totaltransmittance, field uniformity, and uniform pupil irradiance.

[0015] As a solution to these and other object the invention, accordingto one formulation, provides a projection lens for imaging a patternarranged in an object plane onto an image plane employingelectromagnetic radiation from the extreme-ultraviolet (EUV) spectralregion, wherein several imaging mirrors that have reflective coatingsand define an optical axis of the projection lens are arranged betweenthe object plane and the image plane, wherein at least one of thosemirrors has an acentric, graded, reflective coating that has afilm-thickness gradient that is rotationally symmetric with respect to acoating axis, wherein that coating axis is acentrically arranged withrespect to the optical axis of the projection lens.

[0016] The acentricity or eccentricity of a graded, rotationallysymmetric, reflective coating with respect to the optical axis of theentire system provided by the invention yields an additional degree offreedom for optimizing the optical characteristics of the projectionlens that is lacking in conventional systems, where due account shouldbe taken of the fact that the design, or optical layout, of anEUV-projection system may be roughly segregated into two, consecutive,stages. The first stage is optimizing the layout and designs of theuncoated mirror substrates using a computer and suitable optical-designsoftware, duly allowing for the traditional criteria, such as wavefrontaberrations, distortion, assembly conditions, fabrication conditions,etc. Their reflective coatings are then computed and the designrecomputed, duly taking account of their reflective coatings. Thecoatings are effectively “stuck onto” the uncoated substrates, whichgenerally results in imaging performance that is much worse than that ofthe system with uncoated optics, which, as a rule, will be intolerableunless the system is subsequently reoptimized.

[0017] Among other things, that reoptimization should take account ofwavefront aberrations and wavefront apodizations caused by thereflective coatings. The competing effects involved are primarily totalsystem transmittance and field uniformity. It has been found that thesekey properties are usually affected in opposite manners by the sorts ofdesign modifications available. Good compromise solutions that provideadequate total transmittance combined with adequate, field uniformitymay be particularly favorably obtained using acentric, rotationallysymmetric, reflective coatings, where it has generally been found thatproviding acentric, graded, reflective coatings is particularlybeneficial to total transmittance. Field uniformity, on the other hand,is benefited by reflective coatings that are centered on the opticalaxis.

[0018] Angular-range computations for the individual reflectivesurfaces, from which, in particular, the area of each mirror that isactually utilized (their “footprints”), the average angle of incidenceat every point on their surface and the angular bandwidths, or ranges ofangles of incidence, at each point on their surfaces may be derived,usually serve as the starting point for this reoptimization. Theparticularly important items of that data are the average angles ofincidence and the ranges of angles of incidence (angular bandwidths).

[0019] Since the primary purpose of the reflective coatings employed onEUV-systems is reflecting as much of incident electromagnetic radiationas possible, designs may be optimized for maximum reflectance, where theaverage angles of incidence at every point obtainable from angular-rangecomputations may be called upon and used as a basis for computing thefilm thicknesses needed for optimizing reflectance. The manner in whichthis proceeds will be largely determined by the shape of the objectfield, which, in the case of the preferred embodiment, is an annularsegment.

[0020] It has proven beneficial to provide that at least that mirrorthat has the largest range of angles of incidence, i.e., the largestangular bandwidth, has an acentric, graded, reflective coating. Theinvention is based on the recognition that employment of constant filmthicknesses causes enormous reflectance losses on systems where anglesof incidence vary widely over their reflective sections, since filmsthat have constant thicknesses may be optimized for a specially selectedangle of incidence, or for a narrow range of angles of incidences, only.This is particularly a problem on high-aperture systems, e.g., systemsfor which NA>0.2), since angles of incidence on their mirrors arelargely determined by their numerical aperture. Once those mirrors thathave the largest range of angles of incidence have been identified,design modifications, such as shifting a rotationally reflective coatingoff-axis, will allow highly effectively tailoring the system's imagingcharacteristics.

[0021] In the case of a preferred embodiment, the range of angles ofincidence of that mirror that has the largest range of angles ofincidence extends to angles less than 5° to 10° and angles exceeding 10°to 15°. The range of angles of incidence involved may, for example,range from about 1° to about 17°.

[0022] In cases where a reflective system is to be optimized forextremely high total transmittance and field uniformity is eitherunimportant or plays a minor role, it may be beneficial to employ atleast one of those mirrors having the largest range of angles ofincidence has an acentric, graded, reflective coating whosefilm-thickness gradient may be optimized in the radial direction suchthat they will have high reflectance for the radiation employed over thefull range of angles of incidence involved. However, in cases wherecomplex optimizations are involved, it may turn out that employingcoatings that have been optimized for high reflectance will displace thesystem so far from an original local minimum that had been reached thatthat minimum will no longer be automatically locatable, which mightresult in a new design, instead of a reoptimized design.

[0023] Since employing a single, acentric, graded, reflective coating ona reflective system may, in addition to the desired beneficial effects,also adversely affect imaging performance, a preferred embodiment has amirror that has a first, acentric, graded, reflective coating and atleast one other mirror that has a second, acentric, graded, reflectivecoating whose acentricities, film-thickness gradients, etc., have beenadapted to suit one another such that the contributions of theirreflective coatings to certain imaging errors are at least partiallycompensated, where distortion along the cross-scan direction (thex-direction) will be particularly critical, since, for this sort ofdistortion, there is no compensating effect along the orthogonaly-direction due to the scanning. In the case of preferred embodiments,the acentricities of their reflective coatings are thus configured suchthat their coating axis is acentric with respect to the projection lens'optical axis along a y-direction, which, in the case of a scanner,corresponds to the scanning direction.

[0024] It has proven beneficial to provide that the film thicknesses, d,of their rotationally symmetric film-thickness gradients have thefollowing form: $\begin{matrix}{{d = {d_{0}{\sum\limits_{i = 0}^{n}{c_{2i}( {r - r_{0}} )}^{2i}}}},} & (1)\end{matrix}$

[0025] where d₀ is the optimal film thickness for normal incidence (0°angle of incidence), r₀ is the acentricity of the coating axis relativeto the system's optical axis in the x-y plane, r is the current radialcoordinate in the x-y plane, the parameters, c are polynomialcoefficients, and n is an integer. The preferred film-thickness gradientmay thus be described by a second-order polynomial lacking anyodd-powered terms, in particular, lacking a linear term, that may, inthe simplest case, be, for example, a parabola.

[0026] A preferred projection lens that will be described in detail inconjunction with a sample embodiment has six mirrors between its objectplane and image plane. The fifth mirror following its object plane isthat mirror that has the largest range of angles of incidence. In thecase of a system that has been optimized for maximum reflectance, thismirror may have a centered, graded, reflective coating. In the case ofother embodiments, several of their mirrors are provided with centered,graded, reflective coatings that have been adapted to suit one anothersuch that their pupil-irradiance distributions are substantiallyrotationally symmetric. Preferred here are two, and no more than four,such mirrors, since lateral film-thickness gradients are difficult todeposit. At least this fifth mirror is provided with an acentric,graded, reflective coating in order to allow simultaneously optimizingseveral quality criteria, in particular, total transmittance anddistortion.

[0027] Particularly beneficial results are achieved when at least one ofthose mirrors that precede the fifth mirror, for example, the third orfourth mirror, also has an acentric, graded, reflective coating, wherethe axes of rotation of both film-thickness gradients of the cascaded,graded, reflective coatings are acentrically displaced parallel to theoptical axis such that their respective contributions to distortionalong the cross-scan direction at least partially compensate oneanother.

[0028] The foregoing and other characteristics will be apparent, bothfrom the claims and from the description and the drawings, where theindividual characteristics involved may represent characteristics thatare patentable alone or in the form of combinations of subsets thereofin an embodiment of the invention and in other fields.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a schematized longitudinal sectional drawing of anembodiment of an EUV-projection lens that has six mirrors;

[0030]FIG. 2 is a schematized representation of a transmittancedistribution at the exit pupil of that projection lens for a field pointon its symmetry axis for the case where a reflective coating having aconstant film thickness is employed on all of its mirrors;

[0031]FIG. 3 is an associated transmittance distribution at its exitpupil for a field point at the edge of its annular field;

[0032]FIG. 4 is a plot of the transmittance distribution over its imagefield for the case where reflective coatings having constant filmthicknesses are employed;

[0033]FIG. 5 is a schematized representation of a transmittancedistribution at the exit pupil of a projection lens for a field point onits symmetry axis, for the case where one of its mirrors has anacentric, graded, reflective coating;

[0034]FIG. 6 is an associated transmittance distribution at its exitpupil for a field point at the edge of its annular field;

[0035]FIG. 7 is a plot of the transmittance distribution over its imagefield for the case where an acentric, graded, reflective coating isemployed;

[0036]FIG. 8 is a schematized representation of the transmittancedistribution at the exit pupil of a projection lens for a field point onit symmetry axis for the case where one of its mirrors has a centered,graded, reflective coating;

[0037]FIG. 9 is an associated transmittance distribution at its exitpupil for a field point at the edge of its annular field;

[0038]FIG. 10 is a plot of the transmittance distribution over the imagefield of a projection lens for the case where one of its mirrors has acentered, graded, reflective coating;

[0039]FIG. 11 is a schematized representation of the transmittancedistribution at the exit pupil of a projection lens for a field point onit symmetry axis for the case where two of its mirrors have centered,graded, reflective coatings that have been adapted to suit one anotherin order to yield a substantially rotationally symmetricpupil-irradiance distribution;

[0040]FIG. 12 is an associated transmittance distribution at its exitpupil for a field point at the edge of its annular field;

[0041]FIG. 13 is a schematized representation of the transmittancedistribution at the exit pupil of a projection lens for a field point onit symmetry axis for the case where two of its mirrors have centered,graded, reflective coatings;

[0042]FIG. 14 is a schematized representation of the transmittancedistribution at the exit pupil of a projection lens for a field point onit symmetry axis for the case where two of its mirrors have acentric,graded, reflective coatings, where the acentricities of their reflectivecoatings have been adapted to suit one another in order to counteracttheir respective contributions to distortion;

[0043]FIG. 15 is an associated transmittance distribution at its exitpupil for a field point at the edge of its annular field; and

[0044]FIG. 16 is a plot of the transmittance distribution over the imagefield of a projection lens for the case where two of its mirrors haveacentric, graded, reflective coatings for counteracting their respectivecontributions to distortion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] In the following description of the major principles of theinvention, the term “optical axis” shall refer to a straight line or asequence of straight-line segments passing through the paraxial centersof curvature of the optical elements involved, which, in the case ofembodiments described here, consist exclusively of mirrors that havecurved reflecting surfaces. In the case of those examples presentedhere, the object involved is a mask (reticle) bearing the pattern of anintegrated circuit or some other pattern, such as a grating. In the caseof those examples presented here, its image is projected onto a wafercoated with a layer of photoresist that serves as a substrate, althoughother types of substrate, such as components of liquid-crystal displaysor substrates for optical gratings, may also be involved.

[0046] A typical layout of an EUV-projection lens 1 based on a preferredsample embodiment is shown in FIG. 1. It serves to project an image of apattern on a reticle or similar arranged in an object plane 2 onto animage plane 3 aligned parallel to that object plane on a reduced scale,for example, a scale 4:1. Imaging is by means of electromagneticradiation from the extreme-ultraviolet (EUV) spectral region, inparticular, at an operating wavelength of about 13.4 nm. A total of sixmirrors 4-9 that have curved reflecting surfaces, and are thus imagingmirrors, are mutually coaxially arranged between the object plane 2 andimage plane 3 such that they define a common optical axis 10 that isorthogonal to the image plane and object plane. The substrates of thosemirrors 4-9 have rotationally symmetric, aspherical, surface figureswhose symmetry axes coincide with their common physical axis 10. Thissix-mirror system, which has been designed for operation instep-and-scan mode and operates with an off-axis annular field, achievesa numerical aperture, NA, of NA=0.25 for an annular field with typicalfield dimensions of 2 mm×26 mm.

[0047] As may be seen from FIG. 1, light from an illumination system(not shown) that includes a soft-X-ray light source initially strikes areflective mask arranged in the object plane 2 from the side of theobject plane 2 opposite the image. Light reflected by the mask strikes afirst mirror 4 that has a concave reflecting surface facing the objectthat reflects it, slightly narrowed down, to a second mirror 5. Thissecond mirror 5 has a concave reflecting surface facing the first mirror4 that reflects the radiation toward a third mirror 6, in the form of aconvergent beam. This third mirror 6 has a convex reflecting surfacethat reflects the off-axial incident radiation to a fourth mirror 7 thatis utilized in a mirror section situated far away from the optical axisand reflects incident radiation to a fifth mirror 8 arranged in thevicinity of the image plane 3, while forming a real intermediate image11. The latter mirror has a convex reflecting surface facing away fromthe image plane that reflects the incident, divergent, radiation towarda sixth mirror 9 that has a concave reflecting surface facing the imageplane 3 that reflects incident radiation and focuses it on the imageplane 3.

[0048] All reflecting surfaces of the mirrors 4-9 havereflectance-enhancing reflective coatings deposited on them. In the caseof preferred embodiments, these coatings are stacks of, for example,about forty alternating pairs of layers, each of which includes a layerof silicon and a layer of molybdenum.

[0049] Table 1 summarizes the design shown in tabular form, where itsfirst row lists the number of the reflective, or otherwise designatedsurfaces, involved, its second row lists the radius of those surfaces[mm], and third row lists the distance between the respective surfaceinvolved and the next surface [mm]. The algebraic signs of the radiihave been chosen such that a positive sign corresponds to a center ofcurvature of the reflecting surface that lies on the image-plane side.Its fourth through ninth rows, which are designated “A” through “E,”list the aspheric coefficients of the aspherical reflecting surfaces. Itmay be seen that all reflecting surface are spherically curved. Theiraspherical surfaces may be computed using the following equation:

p(h)=[((1/r)h ²)/(1+SQRT(1−(1+K)(1/r)² h ²)]+Ah ⁴ +Bh ⁶+ . . . ,

[0050] where 1/r is their curvature and h is the distance of a point ontheir surface from the optical axis. p(h) thus represents the radialdistance of a point on their surface from the inflection point of theirsurface along the z-direction, i.e., along the optical axis. Theconstants K, A, B, etc., are listed in Table 1.

[0051] The coefficients, C0, C2, xde, and yde, listed in the rows thatfollow describe the film-thickness gradients for the reflective coatingsthat, in the case of a preferred embodiment, are applied to therespective mirrors, in accordance with the formula appearing in Eq. 1,which has been explained above, where r₀={square root}{square root over(xde²+yde²)}.

[0052] The effects that the reflective coatings chosen have on theimaging performance of the projection lens will now be discussed inseveral stages.

[0053] Computerized design of those reflective coatings was conductedunder the boundary condition that each of the coating designs employedshould be allowed to distort the transmitted wavefront only to the pointwhere the entire system would not be displaced from a local minimum ofits characteristics that was found when the system's basic design wasdeveloped for the case of uncoated substrates, where higher-orderwavefront errors hardly occurred at all. The major effects aredistortion and defocusing. In addition to the wavefronts, described by,for example, Zernike coefficients and distortion along the scanningdirection, the y-direction, and the cross-scan direction, thex-direction, the quality criteria that apply to such coating designs arefield uniformity and pupil apodization. System design and fabricationcharacteristics remain virtually unchanged compared to the those of thebasic design with uncoated substrates.

[0054] We shall start off by describing a projection lens, all of whosereflective coatings have constant film thicknesses, where it is usefulto compute an average angle of incidence from the computations of rangesof angles of incidence for all mirrors, where their average angles ofincidence should be computed over their entire utilized surfaces. Theassociated, optimal, film thicknesses are then computed, based on theseglobal average angles of incidence, and inserted into an associatedcoating design in a known manner. The major effect of these uniformlythick films is a constant image offset along the scanning direction,accompanied by a readily recognizable defocusing. This first-order errormay be corrected by a reoptimization. Wavefront aberrations, which aredesignated by their rms-values, were about 20% worse than those fordesigns with uncoated mirrors.

[0055]FIGS. 2 and 3 present plots of the irradiance distribution at theprojection lens' circular exit pupil for two field points, where FIG. 2plots the distribution for a field point lying on the system's symmetryaxis and FIG. 3 plots the distribution for a field point at the edge ofits annular field. The percentages stated designate fractions of theirradiance at the entrance of the projection lens. In keeping with therotational symmetry of the system and the coatings employed, which arerotationally symmetric with respect to any axis parallel to the opticalaxis due to their constant film thicknesses, these two irradiancedistributions differ only in a rotation about the exit pupil's axis. Therotation angle involved results from the location of the field point inthe object plane or image plane.

[0056] These schematic representations show that a pronounced pupilapodization occurs. The irradiance level varies from about 3% to 14%over the pupil. Those areas having differing pupil irradiances areindicated by contours of constant intensity in FIGS. 2 and 3. Thespecial form of these distributions, whose center lies outside the exitpupil, would cause large differences (h−v-differences) between thecritical dimensions (CD-values) for horizontal and vertical features.FIG. 4 schematically depicts the transmittance distribution over thefield. Although the variations along the scanning direction, whichcorrespond to the plot's y-axis, have no significant adverse effects dueto the integrating effect of scanning, the nonuniformities normal to thescanning direction, i.e., the cross-scan direction, or x-direction, areresponsible for CD-variations over the field. However, weighting theirradiation distribution with the irradiance distribution at the reticlewill allow achieving a dynamic uniformity of around 1%, which may beadequate for many types of applications. In the case of the exampleshown, the average transmittance is about 13%, which represents a veryhigh value, in view of simple reflective coatings employed.

[0057] In the case of the design shown in FIG. 1, the fifth mirror 8near the image is the mirror that has the largest variation of averageangle of incidence over its utilized reflective surface, where “angle ofincidence” is defined as the angle between the direction at whichincident radiation is incident and the normal to its reflecting surfaceat the location where radiation strikes that surface. The angles ofincidence involved range from about 1° to 17°. Coating this mirror withcoatings that have a constant thickness will cause relatively highreflection losses.

[0058] A beneficial compromise between high reflectance and acceptablewavefront corrections may be achieved by providing that at least thismirror 8 has a rotationally symmetric, graded, reflective coating whosefilm-thickness gradient may be described by Eq. 1.

[0059] It has been found that in order to significantly improvereflectance it may be sufficient to adapt the film-thickness gradientalong the symmetry axis to suit the average angle of incidence involved.

[0060] This will usually be a linear gradient and may be well-adaptedusing the polynomial of Eq. 1, provided that a corresponding acentricity(r₀≠0) may be tolerated. FIGS. 5 and 6 present plots of intensitydistributions at the exit pupil that yield a high average transmittanceof, for example, 13.7%, with a variance of from about 12% to about 14%,which is thus much less than for the multilayer coatings with constantfilm thicknesses described above. However, the field uniformity shown inFIG. 7, which has a variance of about 2.5%, is somewhat worse than forthe case of coatings with constant film thickness. However, multilayercoatings of this type, which have lateral film-thickness gradients, aresuitable for use in exceptional cases only due to their relatively highdistortion along the cross-scan direction (the x-direction), sincescanning generates no effects that will compensate for this distortion.

[0061] We have been able to show that this distortion along thecross-scan direction is largely attributable to the acentricity of thegraded reflective coating on the fifth mirror 8. This effect can thus bereduced by keeping the acentricity involved small or arranging thegraded reflective coating such that it is rotationally symmetric withrespect to the optical axis. Typical optical characteristics of a designthat employs a centered, graded, reflective coating on the fifth mirrorare shown in FIGS. 8 through 10. It may be seen that pupil apodization,which is about 8% in the vicinity of the pupil's axis and about 14% atthe edge of the pupil, is relatively large. However, it is rotationallysymmetric with respect to the pupil's axis and therefore alsoindependent of field point, which then also immediately yields thenear-perfect uniformity (<0.4%) over the entire field shown in FIG. 10.The total transmittance, 12%, is less than that for the design underconsideration.

[0062] Another embodiment that has been optimized to yield arotationally symmetric pupil apodization will now be described, based onFIGS. 11 through 13. In the case of this embodiment, centered, graded,reflective coatings have been applied to two mirrors, namely, to thefifth mirror 8, which has the largest range of angles of incidence, andto the sixth mirror 9 that is arranged ahead of it in the optical path.The centered film-thickness gradients of these mirrors are adapted tosuit one another such that the pupil apodization is largely rotationallysymmetric, as in the case of the embodiment shown in FIGS. 8 through 10.However, unlike that embodiment, in this case, the pupil apodizationexhibits smaller absolute variances over the exit pupil, whichsignificantly improves the uniformity of the illumination compared tothe case where a single, centered, graded, reflective coating isemployed. As may be seen from FIGS. 11 through 13, the irradiancevariances at the exit pupil, which only range from about 13.4% to about15.6%, are much less than the corresponding variances for an embodimentthat employs just a single, centered, graded, reflective coating (cf.FIGS. 8 through 10). Furthermore, its total transmittance, which isabout 14.7%, is much better than the latter embodiment and its fielduniformity, which is less than 0.4%, is nearly perfect (cf. FIG. 13).Its rotationally symmetric exit pupil is achieved by tailoring thefilm-thickness gradients of the coatings on its third and fifth mirrors.This example shows that employing several, centered, graded, reflectivecoatings whose film-thickness gradients have been suitably adapted tosuit one another will allow achieving substantially rotationallysymmetric pupil-irradiance distributions.

[0063] An embodiment that allows a compromise between totaltransmittance and field uniformity will now be described, based on FIGS.14 through 16. In order to correct for the distortion along thecross-scan direction caused by employment of acentric multilayercoatings mentioned above, it is preferentially foreseen that that thefilm-thickness gradients of the coatings on several, i.e., at last two,mirrors will be acentric and their respective acentricities will havebeen adapted to suit one another such that they partially, or fully,compensate for their contributions to distortion. FIGS. 14 through 16present the characteristics of a design wherein, in addition to thefifth mirror 8 and sixth mirror 9, the third mirror 6, also has anacentric, graded, reflective coating. In addition to correcting fordistortion along the cross-scan direction (residual distortions aretypically less than 1 nm) and acceptable wavefront corrections (typicalrms variances of less than 30 mλ), the system has a very high totaltransmittance of about 13.6% and an acceptable static field uniformity,which is plotted in FIG. 16, of about 1.6%. The field uniformity, asintegrated by scanning motions, should be even less, around 1% or less,and thus much better. It may be seen from FIGS. 14 and 15 that thesecoatings generate a gradient in the irradiance distribution at the pupilthat is somewhat worse than for the case where centered, graded,reflective coatings are employed (cf. FIGS. 8 and 9). The variancesinvolved range from about 9% to 14%. However, the apodization is neithercomplete nor rotationally symmetric, which may adversely affecttelecentricity and the processing window.

[0064] To specialists in the field, it will be clear from theexplanation of the fundamental principles of the invention that, in thecase of projection lenses designed for use in EUV-microlithography,employing suitably applied and, if necessary, combined, acentric,graded, reflective coatings will allow good compromises between totaltransmittance and field uniformity. Particularly beneficial therefor aredesigns that employ several, acentric, graded, reflective coatings,since employment of suitable relative arrangements of such coatingsallows compensating for their adverse effects on imaging errors, such asdistortion, while largely retaining their good total transmittance. Ifnecessary, any intolerable residual errors may still be eliminated byemploying additional, acentric, graded, reflective coatings. Forexample, an acentric grading may be applied to the first mirror in orderto minimize the acentricity at the pupil recognizable in FIGS. 14 and15, without significantly reducing total transmittance

[0065] The above description of the preferred embodiments has been givenby way of example. From the disclosure given, those skilled in the artwill not only understand the present invention and its attendantadvantages, but will also find apparent various changes andmodifications to the structures and methods disclosed. It is sought,therefore, to cover all changes and modifications as fall within thespirit and scope of the invention, as defined by the appended claims,and equivalents thereof. TABLE 1 Surface No. Object M1 M2 M3 M4 M5 M6Image Radius −10704.6665 1058.26338 355.429333 565.293287 549.218427535.96002 Distance 763.156811 −508.895688 592.994217 −263.021453857.514737 −437.185791 481.268511 K 0.129826 −0.000242 0.000328 0.7993550.000227 0.000006 A 5.01967E−10 −8.68517E−11 −8.94789E−10 −6.05680E−105.28599E−09 6.69253E−11 B −3.60955E−15 −8.37923E−16 1.08954E−14−1.14820E−15 1.32773E−13 3.07601E−16 C 4.75929E−20 −7.38993E−21−1.55248E−18 −3.64576E−20 −2.91744E−18 1.31588E−21 D −1.15371E−24−2.26675E−25 1.19824E−22 2.50168E−25 6.32401E−22 1.28688E−27 E2.35510E−29 −8.68225E−30 −3.89134E−27 −1.67219E−30 −6.82763E−267.45365E−32 C0 1.005E+00 1.007E+00 1.577E+00 1.010E+00 1.035E+001.002E+00 C2 0.000E+00 0.000E+00 −5.062E−08 0.000E+00 −6.996E31 060.000E+00 xde 0 0 0 0 0 0 yde 0 0 3159.89 0 14.5538 0

What is claimed is:
 1. A projection lens for imaging a pattern arrangedin an object plane onto an image plane employing electromagneticradiation from the extreme-ultraviolet (EUV) spectral region, whereinseveral imaging mirrors having reflective coatings and defining anoptical axis of the projection lens are arranged between the objectplane and image plane, wherein at least one of those mirrors has anacentric, graded, reflective coating having a film-thickness gradientthat is rotationally symmetric with respect to a coating axis, andwherein that coating axis is acentrically arranged with respect to theoptical axis of the projection lens.
 2. A projection lens according toclaim 1, wherein the mirrors are configured and arranged with respect toone another such that every mirror may be irradiated over a range ofangles of incidence that is characteristic of that particular mirror,wherein at least that mirror having the largest range of angles ofincidence has an acentric, graded, reflective coating.
 3. A projectionlens according to claim 2, wherein the angle of incidence on that mirrorhaving the largest range of angles of incidence ranges fromapproximately 1° to approximately 17°.
 4. A projection lens according toclaim 1, wherein one of the mirrors has a first, acentric, graded,reflective coating and at least one other mirror has a second, acentric,graded, reflective coating, wherein those first and second acentric,graded, reflective coatings are adapted to suit one another such thattheir contributions to imaging errors, in particular, to distortion, areat least partially compensated.
 5. A projection lens according to claim4, wherein the acentric, graded, reflective coatings are arrangedrelative to one another such that their contributions to distortioncompensate for one another over a cross-scanning direction that isorthogonal to a scanning direction.
 6. A projection lens according toclaim 1, wherein a scanning direction and a cross-scanning directionthat is orthogonal thereto are defined, and wherein at least oneacentric, graded, reflective coating is acentric in the scanningdirection.
 7. A projection lens according to claim 1, wherein more thanfive mirrors are arranged between the object plane and image plane.
 8. Aprojection lens according to claim 1, wherein six mirrors are arrangedbetween the object plane and image plane.
 9. A projection lens accordingto claim 1, wherein all mirrors are coaxial.
 10. A projection lensaccording to claim 7, wherein at least the fifth mirror that follows theobject plane has an acentric, graded, reflective coating.
 11. Aprojection lens according to claim 7, wherein the fourth and fifthmirrors that follow the object plane have acentric, graded, reflectivecoatings.
 12. A projection lens according to claim 7, wherein the thirdand fifth mirrors that follow the object plane have acentric, graded,reflective coatings.
 13. A projection lens according to claim 1, whereinthe film thicknesses, d, of the acentric, graded, reflective coatingshave a rotationally symmetric gradient given by:$d = {d_{0}{\sum\limits_{i = 0}^{n}{c_{2i}( {r - r_{0}} )}^{2i}}}$

where d₀ is a film thickness optimized for normal incidence, r₀ is anacentricity, r is a radial coordinate in the x-y plane, and c is acoefficient.
 14. A projection lens according to claim 1, wherein theprojection lens has an image-side numerical aperture, NA, given byNA≧0.15.
 15. A projection lens according to claim 1, wherein theprojection lens has an image-side numerical aperture, NA, given byNA≧0.2.
 16. A projection lens according to claim 1, wherein the mirrorsare configured and arranged such that an intermediate image is formedbetween the object plane and image plane.
 17. A projection lens forimaging a pattern arranged in an object plane onto an image planeemploying electromagnetic radiation from the extreme-ultraviolet (EUV)spectral region, wherein several imaging mirrors having reflectivecoatings and defining an optical axis of the projection lens arearranged between the object plane and image plane, wherein those mirrorsare configured and arranged with respect to one another such that everymirror may be irradiated over a range of angles of incidence that ischaracteristic of that particular mirror, wherein at least that mirrorhaving the largest range of angles of incidence has an acentric, graded,reflective coating that has a film-thickness gradient that isrotationally symmetric with respect to a coating axis, and wherein thatcoating axis is substantially coincident with the optical axis of theprojection lens.
 18. A projection lens according to claim 17, whereinthe film-thickness gradient of the graded, reflective coating isoptimized to yield a pupil-irradiance distribution that is substantiallyrotationally symmetric.
 19. A projection lens according to claim 17,wherein several mirrors have acentric, graded, reflective coatings,wherein the gradients in the film thicknesses of their reflectivecoatings are adapted to suit one another such that they yield apupil-irradiance distribution that is substantially rotationallysymmetric.
 20. A projection lens according to claim 17, wherein theprojection lens has an image-side numerical aperture, NA, given byNA≧0.15.
 21. A projection lens according to claim 17, wherein theprojection lens has an image-side numerical aperture, NA, given byNA≧0.2.
 22. A projection exposure system for use in microlithographyemploying electromagnetic radiation from the extreme-ultraviolet (EUV)spectral region, comprising an illumination system and a projection lensfor imaging a pattern arranged in an object plane onto an image plane,the projection lens comprising several imaging mirrors having reflectivecoatings and defining an optical axis of the projection lens and beingarranged between the object plane and image plane, wherein at least oneof those mirrors has an acentric, graded, reflective coating having afilm-thickness gradient that is rotationally symmetric with respect to acoating axis, and wherein that coating axis is acentrically arrangedwith respect to the optical axis of the projection lens.
 23. A methodfor fabricating semiconductor devices, or other types of microdevices,comprising the following steps: providing a mask having a prescribedpattern; illuminating the mask with electromagnetic radiation from theextreme-ultraviolet (EUV) spectral region; and projecting an image ofthe pattern onto a photosensitive substrate arranged in the image planeof a projection lens using a projection lens comprising several imagingmirrors having reflective coatings and defining an optical axis of theprojection lens and being arranged between the object plane and imageplane, wherein at least one of those mirrors has an acentric, graded,reflective coating having a film-thickness gradient that is rotationallysymmetric with respect to a coating axis, and wherein that coating axisis acentrically arranged with respect to the optical axis of theprojection lens.